Originally posted by Sinter Klaas
The Sahara has not always been a desert you know.
There is evidence that shows it was a green heaven during man's existence.
This postulates that there is no single "cradle", but several independent developments of civilization, of which the Near Eastern Neolithic was the first.
The climate of modern Antarctica is extreme. Located over the South Pole and in total darkness for six months of the year, the continent is covered by glacial ice to depths in excess of 3 km in places. Yet this has not always been the case. 50 Ma ago, even though Antarctica was in more or less the same position over the pole, the climate was much more temperate – there were no glaciers and the continent was covered with lush vegetation and forests. So how did this extreme change come about?
The modern climate of Antarctica depends upon its complete isolation from the rest of the planet as a consequence of the Antarctic Circumpolar Current that completely encircles Antarctica and gives rise to the stormy region of the Southern Ocean known as the roaring forties. The onset of this current is related to the opening of seaways between obstructing continents. Antarctica and South America were once joined together as part of Gondwana and were the last parts of this original supercontinent to separate. By reconstructing continental positions from magnetic and other features of the sea floor in this region, geologists have shown that the Drake Passage opened in three phases between 50 Ma and 20 Ma, as illustrated in Figure 32. At 50 Ma there was possibly a shallow seaway between Antarctica and South America, but both continents were moving together. At 34 Ma the seaway was still narrow, but differential movement between the Antarctic and South American Plates created a deeper channel between the two continents that began to allow deep ocean water to circulate around the continent. Finally, at 20 Ma there was a major shift in local plate boundaries that allowed the rapid development of a deep-water channel between the two continental masses.
Today, the Antarctic Circumpolar Current is the strongest deep ocean current and its strength is responsible for the ‘icehouse’ climate that grips the planet. The opening of the Drake Passage had both a local and a global effect, initially cooling the climate of Antarctica from temperate to cold and ultimately playing an important role in the change from global ‘greenhouse’ conditions 50 Ma ago to the global ‘icehouse’ of today.
Sea-temperatures vary from about −2 to 10 °C (28 to 50 °F). Cyclonic storms travel eastward around the continent and frequently become intense because of the temperature-contrast between ice and open ocean. The ocean-area from about latitude 40 south to the Antarctic Circle has the strongest average winds found anywhere on Earth. In winter the ocean freezes outward to 65 degrees south latitude in the Pacific sector and 55 degrees south latitude in the Atlantic sector, lowering surface temperatures well below 0 degrees Celsius; at some coastal points intense persistent drainage winds from the interior keep the shoreline ice-free throughout the winter.
huge icebergs with drafts up to several hundred meters; smaller bergs and iceberg fragments; sea ice (generally 0.5 to 1 meter thick) with sometimes dynamic short-term variations and with large annual and interannual variations; deep continental shelf floored by glacial deposits varying widely over short distances; high winds and large waves much of the year; ship icing, especially May-October; most of region is remote from sources of search and rescue
There is also evidence this was the case 250.000 years ago and according to the sources I found even as recent as 2000 to 4000 years ago.
How do we know what would cause the Ocean Conveyor Belt to stop? The Younger Dryas period of colder temperatures, which occurred about 12 thousand years ago, occurred because of the shutdown of the Ocean Conveyor Belt (Joyce 2007). In An Inconvenient Truth, Al Gore states that the ocean overturning ceased because of the melted glaciers in modern-day Canada and the United States (now the Great Lakes), which spilled over into the North Atlantic, which is a critical region of overturning
The analyses of ocean-floor sediments deposited recently by melting Antarctic ice sheets reveal that these ice sheets are only about 2,000 years old.
Ocean-floor sediments drilled from Antarctic regions recently covered by ice shelves suggest that those shelves were only 2,000 years old. This finding could compel scientists to reassess whether the current destruction of polar ice is due primarily to human-caused global warming.
Their analysis suggests that from about 2,000 to 5,000 years ago, much of the channel was seasonally open water.
Sediment cores have been collected from the ocean bottom in an area, just north of Larsen B, exposed by ice-shelf disintegrations in the early 1990s. The cores indicate that the shelf there was only about 2,000 years old (SN: 9/8/01, p. 150: www.sciencenews.org...). However, a preliminary analysis of sediment layers in cores taken in December 2001 from seafloor near Larsen B suggests that this shelf has been in place for more than 12,000 years, says Eugene W. Domack, a marine geologist at Hamilton College in Clinton, N.Y. He'll present those results at an international workshop on Antarctic climate variability hosted by his college next week.
It appears that before and after this period, the channel remained closed. The period when the channel was open coincides with a period of local warming supported by data gathered from land-based studies of lake sediments and ancient, abandoned penguin rookeries. With the return of colder conditions about 1900 years ago, the Prince Gustav ice shelf reformed until its recent retreat.